Re: CALL_USED_REGISTERS per function basis

[Michael Meissner <meissner at linux dot vnet dot ibm dot com>]

On Thu, Aug 05, 2010 at 03:48:55PM +0200, Claudiu Zissulescu wrote:
> Hi,
> 
> I want to use a different CALL_USED_REGISTER set per individual
> function. The issue here is to inform a caller about the callee
> CALL_USED_REGISTERS and save/restore at caller level the possible
> altered registers.  This should reduce the number of saved/restored
> operation in a function.
> 
> Can someone give me some pointers in this direction?

In the x86 they use TARGET_EXPAND_TO_RTL_HOOK which points to the
ix86_maybe_switch_abi function.

That function is:
/* MS and SYSV ABI have different set of call used registers.  Avoid expensive
   re-initialization of init_regs each time we switch function context since
   this is needed only during RTL expansion.  */
static void
ix86_maybe_switch_abi (void)
{
  if (TARGET_64BIT &&
      call_used_regs[SI_REG] == (cfun->machine->call_abi == MS_ABI))
    reinit_regs ();
}

Going beyond the above, about 2 years ago, I and another programmer wrote the
function specific support that allows you to use attributes and pragmas to say
a particular function is compiled with non-standard options.  My intention was
that you could declare one function normally, another with SSE2 support, and a
third with SSE4 support, and that at runtime you could switch to use the
function that supported the instruction set.  This was done via the
TARGET_SET_CURRENT_FUNCTION callback, which in the x86 case is
ix86_set_current_function.  This does the target_reinit and eventually
reinit_regs.

Now, unfortunately, I've been away from the code for about 2 years, and I don't
know whether it has bit-rotted or not.

-- 
Michael Meissner, IBM
5 Technology Place Drive, M/S 2757, Westford, MA 01886-3141, USA
meissner@linux.vnet.ibm.com